Alcohols, in particular, 2-propanol (isopropyl alcohol-IPA), are being increasingly utilized in various industries as solvents and cleaning agents. Purification of alcohol streams when contaminated by water at certain concentrations become difficult due to the formation of azeotropic mixtures, wherein the equilibrium vapor and liquid compositions are the same. Such azeotropic mixtures of alcohol and water cannot be separated by normal distillation but only through complicated processes. Frequently, an additional substance is added to break down this azeotropic mixture. This additional substance must subsequently be completely removed and recovered from both product streams. An easy, efficient recovery and reuse of alcohols is imperative in view of the process economics and to meet environmental regulations. Prior art processes using other memmbranes and gels did not have the desirable properties of the compositions of this invention.
Pervaporation and vapor permeation are membrane based operations, in which water-free alcohols can be produced in a simple and energy efficient way. In pervaporation, water from a contaminated organic stream is preferentially transported across a thin membrane film. The source side of the membrane is wetted with the organic liquid. A vacuum or a sweep gas is used on the sink side of the membrane. The water is collected from the sink side by condensation.
Vapor permeation is similar to pervaporation with one major difference-vapors instead of liquid contact the source side of the membrane. In contrast to other membrane filtration processes, pervaporation/vapor permeation works according to a solution diffusion mechanism. The membrane itself must be non-porous for pervaporation to work.
In microfiltration or ultrafiltration, for example, porosity is the key to preferential transport, and the flux rate depends upon molecular size. In pervaporation/vapor permeation, molecular interaction between membrane and separated species is the determining factor rather than the molecular size. The main component of the pervaporation/vapor permeation process is the membrane material which determines the permeation and selectivity and hence the separation properties of the process.
For pervaporation and/or vapor permeation to be economical and efficient, ultra thin, non-porous, hydrophilic films of appropriate polymer need to be deposited onto a highly porous support matrix. Such a combination will provide high throughput along with good mechanical stability and thus result in achieving the desired separation using minimum membrane area. Since water needs to be transported across the membrane, a high trans membrane flow hydrophilic membrane must be used. The trans membrane flow is a function of the composition of the feed. It is usually expressed as permeate amount per membrane area and unit time, e.g. kg/m2-hr, for the better permeating component.
A further essential criterion for the suitability of the pervaporation membrane is its chemical and thermal stability. To obtain a high transmembrane flow and a sufficient driving force, it is necessary to operate the pervaporation process at the highest possible temperatures. This means that the membrane will be in contact with a feed mixture at high temperature which has a high concentration of organic components.
To achieve an economical lifetime of the membranes all components of the membrane must be durable under aggressive conditions. The most common dehydration membrane reported in literature for use in pervaporation processes is prepared from polyvinyl alcohol (PVA).